How to Mirror After Starting Sculpt with MultiRes Modifier
This may be out of date, but quick tip - if you mirror a mesh by scaling -1, you can then reverse the normals via 'recalculate outside' or 'flip normals'
see:
https://docs.blender.org/manual/en/latest/modeling/meshes/editing/mesh/normals.html#recalculate
https://docs.blender.org/manual/en/latest/modeling/meshes/editing/mesh/normals.html#flip
You may also be able to work around this by changing the order of the modifiers in the stack.